In general, in an image reading apparatus, a cylindrically shaped lamp is disposed in a widthwise direction of an original, and light produced from the lamp is applied to the original. Then, the light reflected by the original or the light transmitted through the original is read by an image sensor, such as a CCD image sensor.
As lamps which are used for image reading apparatuses and various other apparatuses, those of various configurations are conventionally known.
For example, a hot-cathode tube of a so-called both-end electrode type is known in which, as shown in FIG. 16, a pair of electrodes 52a and 52b serving as hot cathodes are disposed at both ends of a gas-filled tube 51 in which a gas is filled. Reference numeral 53 denotes a fluorescent material applied to an inner peripheral surface of the gas-filled tube 51. As is well known, the hot cathode refers to an electrode which generates heat upon being energized, and emits thermions.
In addition, a cold-cathode tube of a so-called both-end electrode type is also known in which, as shown in FIG. 17, a pair of electrodes 54a and 54b serving as cold cathodes are disposed at both ends of a gas-filled tube 51. As is well known, the cold cathode refers to an electrode which emits electrons when a strong electric field is applied thereto.
Further, a cathode tube of a so-called outer-surface electrode type is also known in which, as shown in FIG. 18, a pair of elongated electrodes 55a and 55b are disposed substantially over entire longitudinal regions of the outer peripheral surface of a gas-filled tube 51 in such a manner as to face each other. Under the circumstances, the electrodes 55a and 55b are generally configured as cold cathodes, but in a case where the electrodes 55a and 55b can be configured as hot cathodes, such an arrangement may be adopted.
In the various types of lamps described above, a rare gas, such as neon (Ne) gas or xenon (Xe) gas, or mercury (Hg) gas is filled in the gas-filled tube 51, depending on applications. If an examination is made of the manner of change in the quantity of emission of light at a time when the ambient temperature is changed with respect to, for example, a lamp using the mercury gas and a lamp using the xenon gas, results such as those shown in FIG. 11 are obtained. As is apparent from the results, the lamp using the xenon gas excels over the lamp using the mercury gas in terms of the stability in the-quantity of light with respect to the change in the ambient temperature. Accordingly, the xenon gas is used widely for the lamps of image reading apparatuses. In addition, since the neon gas also excels in the stability of the quantity of light, the neon gas is also used widely for the lamps of image reading apparatuses.
If the change over time of the quantity of light emitted from the lamp is observed, results such as those shown in FIG. 12 are obtained. In the graph shown in the drawing, the rate of change in the quantity of light is taken as the X-axis, the time is taken as the Y-axis, and the axial direction, i.e., the horizontal scanning direction, of a lamp 56 is taken as the Z-axis. It is now assumed that the quantity of light immediately after the lamp 56 is lit up is uniformly 100% in its axial direction, i.e., in its longitudinal direction, as shown by reference character A. After the lapse of an appropriate time t subsequent to lighting, the overall quantity of light from the lamp 56 declines as shown by reference character B1, and the quantity of light declines more in a central portion in the longitudinal direction than in opposite end portions. In this description, a change .delta.1 in the quantity of light in the X-direction when 100% of the quantity of light immediately after lighting-up is set as a reference will be referred to as a rate of change in the quantity of light. In addition, when one line of distribution of the quantity of light is viewed at an arbitrary point of time, the difference .delta.2 between a maximum rate of change in the quantity of light (normally in a central portion) and a minimum rate of change in the quantity of light (normally in opposite end portions) in that line of distribution of the quantity of light will be referred to as a difference in the rate of change in the quantity of light.
For example, if a comparison is made among the three types of lamps shown in FIGS. 16 through 18, i.e., the cathode tube of the outer-surface electrode type (FIG. 18), the hot-cathode tube of the both-end electrode type (FIG. 16), and the cold-cathode tube of the both-end electrode type (FIG. 17), the quantity of emission of light is the largest in the case of the cathode tube of the outer-surface electrode type (FIG. 18), the next largest is the hot-cathode tube of the both-end electrode type (FIG. 16), and the smallest is the cold-cathode tube of the both-end electrode type (FIG. 17). That is, with respect to the quantity of emission of light, it can be said that
The cathode tube of the outer-surface electrode type (FIG. 18) &gt;the hot-cathode tube of the both-end electrode type (FIG. 16) &gt;the cold-cathode tube of the both-end electrode type (FIG. 17) Specifically, the cathode tube of the outer-surface electrode type (FIG. 18) has a quantity of emission of light which is about three times greater than that of the cold-cathode tube of the both-end electrode type (FIG. 17).
On the other hand, both the rate of change .delta.1 in the quantity of light and the difference .delta.2 in the rate of change in the quantity of light are relatively small in the case of the hot-cathode tube of the both-end electrode type (FIG. 16) and the cold-cathode tube of the both-end electrode type (FIG. 17). In contrast, both the rate of change .delta.1 in the quantity of light and the difference .delta.2 in the rate of change in the quantity of light are considerably large in the case of the cathode tube of the outer-surface electrode type (FIG. 18).
Since the rate of change 61 in the quantity of light and the difference 62 in the rate of change in the quantity of light are small in the case of the hot-cathode tube of the both-end electrode type (FIG. 16) and the cold-cathode tube of the both-end electrode type (FIG. 17) as described above, in a case where these cathode tubes are used for ordinary image reading apparatuses, no problems are encountered in practical use from the standpoint of the change in the quantity of light. However, since the quantity of emission of light per se is small in the case of these lamps, there are cases where it is impossible to obtain clear read images. In addition, even if the variations in the quantity of light are small, and no problems are encountered in practical use, it can be said with respect to ordinary image reading apparatuses, and if such cathode tubes are used as lamps for high-quality image input apparatuses as in the case of reading films in applications for publication, the reproducibility of reading results still declines due to variations in the quantity of light.
The cathode tube of the outer-surface electrode type (FIG. 18) has a large quantity of emission of light, and is therefore suitable for reading clear images. However, since this lamp has a considerably large rate of change .delta.1 in the quantity of light and a considerably large difference .delta.2 in the rate of change in the quantity of light, if this lamp is used as the lamp for the image reading apparatus, the characteristics of read images deteriorate due to the change over time in the quantity of light. Specifically, there arises the problem that the quantity of light changes substantially between the start of reading of the original and the end of reading, the reproduced image becomes gradually dark, and a central portion, in particular, becomes dark.
Accordingly, as a countermeasure for overcoming such a problem, there has been a demand to lower the rate of change .delta.1 in the quantity of light and the difference .delta.2 in the rate of change in the quantity of light in the lamp.
Meanwhile, Japanese Patent Application Laid-Open No. 123214/1995 discloses an image input apparatus in which, as shown in FIGS. 19 and 20, heat is insulated by maintaining heat which is generated by a fluorescent lamp 92 itself during the emission of light by attaching a heat insulating material 93 to the fluorescent lamp 92 in close contact therewith so as to improve the lighting-up rise characteristic, and the fluorescent lamp 92 is controlled to an appropriate quantity of light by detecting the illuminance by a photosensor 98. Incidentally, reference numeral 91 denotes an original-reading background plate; 94, a mirror; 95, a slit; 96, a lens; 97, a CCD image sensor; and 99, an original which is fed in the direction of arrow. The conventional technique disclosed in this publication is aimed at improving the lighting-up rise characteristic of the lamp, and gives no disclosure as to the reduction of both the rate of change .delta.1 in the quantity of light and the difference .delta.2 in the rate of change in the quantity of light to satisfactory levels.